The field of the invention relates generally to accelerometry. More specifically, the field of the invention relates to a microelectromechanical accelerometer. In particular, the field of the invention relates to a microelectromechanical accelerometer fabricated from single crystal silicon, with improved performance qualities, for use in automotive and related applications, and a manufacturing method for making such accelerometers at low cost.
There is a need for automotive safety systems to collect more information about vehicle dynamics and external forces acting on the vehicle in order to make intelligent decisions as to what actions, if any, need to be taken to maintain safe vehicle operation. Collecting such information is the role of sensors, such as accelerometers, force sensors, pressure sensors, and the like. With presently available sensor technologies, only a limited number of sensors can be utilized in a vehicle before their cost becomes prohibitively high. What is required is a new, high-performance, low-cost technology for automotive sensors. Silicon-based devices and microelectronics-style manufacturing techniques are anticipated to be required to meet the price-performance objectives of automotive sensors in the future. See G. A. MacDonald, "A Review of Low Cost Accelerometers for Vehicle Dynamics," Sensors and Actuators A21-A23 (1990), pp. 303-307; and Robert E. Sulouff, Jr., "Silicon Sensors for Automotive Applications," Proc. 6th Int. Conf. Solid-State Sensors and Actuators (Transducers '91), San Francisco, Calif., Jun. 24-28, 1991, pp. 170-176.
There are numerous applications for accelerometers in automobiles, including airbag deployment (front, rear, and side impact), anti-lock brake systems, roll detection, angular rate accelerometers, electronically controlled suspension systems, steering systems, and collision avoidance systems, to name a few. Each application requires accelerometers which operate in different ranges of acceleration (from as little as 10.sup.-6 g to as much as 500 g) and bandwidth, yet all with stringent requirements on reliability, operating environment, self testability, and cost. See G. A. MacDonald, "A Review of Low Cost Accelerometers for Vehicle Dynamics," Sensors and Actuators A21-A23 (1990), pp. 303-307; and Robert E. Sulouff, Jr., "Silicon Sensors for Automotive Applications," Proc. 6th Int. Conf. Solid-State Sensors and Actuators (Transducers '91), San Francisco, Calif., Jun. 24-28, 1991, pp. 170-176.
Introduction of new technology to automotive applications is primarily driven by price-performance considerations. Although more intelligent safety systems are desired, the cost of those systems must continuously drop while their performance improves. If improving the system's performance requires more sensors, the price of individual sensors and their associated electronics must be correspondingly lower. Assuming a ten to twenty times increase in the number of sensors (not unreasonable considering a fully active suspension is predicted to require ten accelerometers alone) and a ten times reduction in the cost of the overall safety system, then the sensors themselves must be produced for less than one-hundredth (1/100) their current price. As a concrete example, high performance piezoelectric quartz accelerometers which could be utilized in these automotive applications currently retail for $300. The automobile industry predicts that such sensors will not be incorporated into production vehicles until a technology can be found which can supply the desired sensor for $2 to $3 per unit.
An excellent example of the commercial reality discussed above can be found in automotive airbag systems. Over the past five years a concerted effort in the industry has been made to develop a new airbag deployment subsystem costing less than one-tenth that of the current technology. Substantial investment in time and research funds have been made and functioning devices have been delivered to potential customers. However, these devices cannot meet the cost targets set out for them by the automotive industry. Hence, despite much promise, there is at present virtually no use of these new accelerometer devices in production airbag systems. The following discussion of accelerometers, and particularly micromachined accelerometers, helps to explain why these accelerometers have high cost and have not achieved widespread use in automotive applications.
An accelerometer in general is a device which senses an externally-induced acceleration. There are three major components to an accelerometer, as shown in FIG. 1. Typically, a sense element is a mass of some sort which moves in response to an applied acceleration vector. This is referred to as a mass, proof mass or seismic mass. The proof mass is held in its resting position by a spring. Some form of displacement transducer is used to measure the amount of motion the proof mass makes in response to an applied acceleration. This is then converted into an electrical output signal and may include signal conditioning electronics to provide a strengthened signal for accurate measurement of the displacement. The output signal from signal conditioning electronics then may be used by additional electronic control circuitry to determine how to respond to the detected acceleration. For example, a charge may be activated for deploying an airbag in response to a sensed acceleration vector above a pre-determined threshold. See Ernest O. Doebelin, Measurement Systems: Application and Design, (McGraw-Hill, New York, 1990), Chapter 4.6, incorporated herein by reference.
Present generation production airbag deployment sensors utilize physically "large" mechanical devices, such as a metallic ball held between the poles of a permanent magnet, as the accelerometer to detect impact (deceleration) of sufficient magnitude to signal deployment of the airbag, typically an impact in excess of 50 g (490 m/s.sup.2). See, for example, U.S. Pat. No. 5,098,122. This type of conventional accelerometer has severe disadvantages in terms of cost, reliability, sensitivity, and self-testing ability. Thus, there is a compelling need for an alternative accelerometer technology for an airbag deployment system which provides low cost, reliable and ultra sensitive operation along with self testing capability.
Moreover, there are numerous other applications for accelerometers in automobiles such as active suspension, anti-lock braking, and active steering, and the necessary broad range of operating characteristics for active steering which cannot be met by current "large" mechanical accelerometer technology Solid-state accelerometers based on the piezoelectric effect in many cases have been implemented in an attempt to meet the performance requirements of these additional applications. However, such conventional piezoelectric accelerometers, are too expensive and/or physically too large to be practical for implementation in automobiles. See, for example, U.S. Pat. No. 4,945,765, which notes that these large accelerometers can be several cubic inches in size and weigh a pound.
The emerging technology of micromechanical systems (MEMS) has created an entirely new approach to accelerometers (see, for example, Janusz Bryzek, Kurt Petersen, and Wendell McCulley, "Micromachines on the March," IEEE Spectrum, May 1994, pp. 20-31, and Lee O'Connor, "MEMS: Micromechanical Systems," Mechanical Engineering, February 1992, pp. 40-47). Numerous patents have been issued for a variety of micromechanical accelerometers over the past fifteen years (for example, U.S. Pat. Nos. 4,483,194; 4,553,436; 4,736,629; 4,945,765; 5,126,812; 5,249,465; and 5,345,824). The earliest of these patents, as well as research papers from the late '70's made reference to the potential application of micromechanical accelerometers in automotive applications based on the potential of MEMS to meet both the cost and performance requirements described above. Yet as far as is known at this time, few MEMS accelerometers are used in automotive applications.
MEMS utilizes microelectronic processing techniques to reduce mechanical components to the scale of microelectronics. In some cases, it is even possible, although quite difficult, to place both the mechanical components and electronics onto a common silicon chip. MEMS offers the opportunity for integrating mechanical sensor elements and their associated signal processing electronics onto a single chip in a common manufacturing process, if a viable process for this integration can be found. This integrated approach is in stark contrast to existing technology in which separate manufacturing processes and facilities must be used to fabricate the mechanical components and the electronic components. Those individual components then must be assembled together in the final package. This results in manufacturing complexity and greatly increases the cost of the final product. Consequently, MEMS offers the potential for substantial reductions in size and weight, and tremendous improvements in cost, performance, and reliability when compared to existing technology.
Two principal fabrication technologies are used to create MEMS devices: bulk and surface micromachining. See, for example, U.S. Pat. Nos. 4,736,629 and 5,345,824; Theresa A. Core, W. K. Tsang, and Steven J. Sherman, "Fabrication Technology for an Integrated Surface-Micromachined Sensor," Solid State Technology, October 1993, pp. 39-47; and Wolfgang Kuehnel and Steven Sherman, "A surface micromachined silicon accelerometer with on-chip detection circuitry," Sensors and Actuators A 45 (1994), pp. 7-16; Frank Goodenough, "Airbags Boom When IC Accelerometer Sees 50 g," Electronic Design, Aug. 8, 1991, pp. 45-56; Lynn Michelle Roylance and James B. Angell, "A Batch-Fabricated Silicon Accelerometer," IEEE Trans. Electron Dev. ED-26 (1979) pp. 1911-1917; Lj. Ristic, D. Koury, E. Joseph, F. Shermansky, and M. Kniffin, "A Two-Chip Accelerometer System for Automotive Applications," Proc. MicroSystem Technologies '94, Berlin, Oct. 19-21, 1994, pp. 77-84; and U.S. Pat. No. 5,249,465.